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Plasmonic nanoswitches could increase computing power a million fold

22 February 2009

Plasmonics — a possible replacement for current microchip technology
— may pave the way for the next generation of computers that operate
faster and store more information than electronically-based systems and
are smaller than optically-based systems, according to a Penn State
University engineer who has developed a plasmonic switch.

"If plasmonics are realized, the future will have circuits as small
as the current electronic ones with a capacity a million times better,"
said Tony Jun Huang, James Henderson assistant professor of Engineering
Science and Mechanics. "Plasmonics combines the speed and capacity of
photonic — light based — circuits with the small size of electronic
circuits."

Currently, electronic circuits can be made very small, but they are
limited by their capacity and the speed that information can travel in
the circuits. Optical circuits send information at the speed of light,
but the size is large, limited by the light's wavelength.

Plasmonics combines the best of electronic and optical circuits and
can transmit electrons and light at the same time using the surface of
the device.

Huang's team created a plasmonic switch from switchable bistable
rotaxanes. Rotaxanes are complex molecules that consist of a dumbbell
shape with a ring or rings encircling the shaft and are sometimes called
molecular machines. The ring can either move from one end of the barbell
to the other or rotate around the shaft. Changes in molecular shape are
the basis of the plasmonic switch.

Computers, in their simplest form, are machines that can say yes or
no multiple times to transfer information. The motion of a molecule can
serve the same purpose as the on off switch on a light.

The researchers attached their molecular machines to gold-coated
nanodiscs fabricated on glass. The machines were attached with disulfide
functional groups. The dumbbell shaped molecules have two areas of the
shaft primed with two different chemicals.

The ring is initially drawn to circle at one primed area. When the
chemical there is oxidized, the ring is repelled and moves to the other
primed area, flipping the switch. The process is reversible, so the ring
returns to its original state to switch on again later.

When the molecule moves, it changes the surface plasmon resonance in
that tiny area of the metal where it is attached. This change in
resonance is what would send the signal on the circuit. The plasmonic
switch that Huang and his team developed is not yet part of a circuit.

"Plasmonic circuits have not yet been achieved," said Huang. "In the
past, the plasmonic devices made were all passive." These devices were
used as light sources, lenses and waveguides.

Huang's switches are activated by a chemical process, however, this
is not the optimal choice for a working circuit.

"We believe that the chemically-driven redox process can be replaced
with direct electrical or optical stimulation, a logical development
that would establish a technological basis for the production of a new
class of molecular-machine-based active plasmonic components for
solid-state nanophotonic integrated circuits with the potential for
low-energy and ultra small operations," the researchers state in a
recent issue of Nano Letters.

In essence, plasmonic devices would allow computers to get faster and
have more memory storage in smaller spaces. Storage of as much as 1,000
movies on a typical USB drive would be possible. Huang suggests that
applications like YouTube, which are very popular but have terrible
resolution, could become places to see high-resolution images.

"We are in the very beginning of this field," said Huang. "Creation
of a plasmonic circuit is probably five years away."

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